From: vbabka@suse.cz (Vlastimil Babka) Date: Thu, 29 Jan 2015 14:13:38 +0100 Subject: [PATCHv2] mm: Don't offset memmap for flatmem In-Reply-To: <20150126155617.GA2395@suse.de> References: <1421804273-29947-1-git-send-email-lauraa@codeaurora.org> <1421888500-24364-1-git-send-email-lauraa@codeaurora.org> <20150122162021.aa861aeb53c22206a19ebbcb@linux-foundation.org> <54C196D0.6040900@codeaurora.org> <54C20EEC.1060809@suse.cz> <20150126155617.GA2395@suse.de> Message-ID: <54CA3202.8020609@suse.cz> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 01/26/2015 04:56 PM, Mel Gorman wrote: > On Fri, Jan 23, 2015 at 10:05:48AM +0100, Vlastimil Babka wrote: >> On 01/23/2015 01:33 AM, Laura Abbott wrote: >>> On 1/22/2015 4:20 PM, Andrew Morton wrote: >>>> >>>> I don't think v2 addressed Vlastimil's review comment? >>>> >>> >>> We're still adding the offset to node_mem_map and then subtracting it from >>> just mem_map. Did I miss another comment somewhere? >> >> Yes that was addressed, thanks. But I don't feel comfortable acking >> it yet, as I have no idea if we are doing the right thing for >> CONFIG_HAVE_MEMBLOCK_NODE_MAP && CONFIG_FLATMEM case here. >> >> Also putting the CONFIG_FLATMEM && !CONFIG_HAVE_MEMBLOCK_NODE_MAP >> under the "if (page_to_pfn(mem_map) != pgdat->node_start_pfn)" will >> probably do the right thing, but looks like a weird test for this >> case here. >> >> I have no good suggestion though, so let's CC Mel who apparently >> wrote the ARCH_PFN_OFFSET correction? >> > > I don't recall introducing ARCH_PFN_OFFSET, are you sure it was me? I'm just > back today after been offline a week so didn't review the patch but IIRC, > ARCH_PFN_OFFSET deals with the case where physical memory does not start > at 0. Without the offset, virtual _PAGE_OFFSET would not physical page 0. > I don't recall it being related to the alignment of node 0 so if there > are crashes due to misalignment of node 0 and the fix is ARCH_PFN_OFFSET > related then I'm surprised. You're right that ARCH_PFN_OFFSET wasn't added by you, but by commit 467bc461d2 which was a bugfix to your commit c713216dee, which did introduce the mem_map correction code, and after which the code looked like: mem_map = NODE_DATA(0)->node_mem_map; #ifdef CONFIG_ARCH_POPULATES_NODE_MAP if (page_to_pfn(mem_map) != pgdat->node_start_pfn) mem_map -= pgdat->node_start_pfn; #endif /* CONFIG_ARCH_POPULATES_NODE_MAP */ It's from 2006 so I can't expect you remember the details, but I had some trouble finding out what this does. I assume it makes sure that mem_map points to struct page corresponding to pfn 0, because that's what translations using mem_map expect. But pgdat->node_mem_map points to struct page corresponding to pgdat->node_start_pfn, which might not be 0. So it subtracts node_start_pfn to fix that. This is OK, as the node_mem_map is allocated (in this very function) with padding so that it covers a MAX_ORDER_NR_PAGES aligned area where node_mem_map may point to the middle of it. Commit 467bc461d2 fixed this in case the first pfn is not 0, but ARCH_PFN_OFFSET. So mem_map points to struct page corresponding to pfn=ARCH_PFN_OFFSET, which is OK. But I still have few doubts: 1) The "if (page_to_pfn(mem_map) != pgdat->node_start_pfn)" sort of silently assumes that mem_map is allocated at the beginning of the node, i.e. at pgdat->node_start_pfn. And the only reason for this if-condition to be true, is that we haven't corrected the page_to_pfn translation, which uses mem_map. Is this assumption always OK to do? Shouldn't the if-condition be instead about pgdat->node_start_pfn not being aligned? 2) The #ifdef guard is about CONFIG_ARCH_POPULATES_NODE_MAP, which is nowadays called CONFIG_HAVE_MEMBLOCK_NODE_MAP. But shouldn't it be #ifdef FLATMEM instead? After all, we are correcting value of mem_map based on page_to_pfn code variant used on FLATMEM. arm doesn't define CONFIG_ARCH_POPULATES_NODE_MAP but apparently needs this correction. 3) The node_mem_map allocation code aligns the allocation to MAX_ORDER_NR_PAGES, so the offset between the start of the allocated map and where node_mem_map points to will be up to MAX_ORDER_NR_PAGES. However, here we subtract (in current kernel) (pgdat->node_start_pfn - ARCH_PFN_OFFSET). That looks like another silent assumption, that pgdat->node_start_pfn is always between ARCH_PFN_OFFSET and ARCH_PFN_OFFSET + MAX_ORDER_NR_PAGES. If it were larger, the mem_map correction would subtract too much and end up below what was allocated for node_mem_map, no? The bug report behind this patch said that first 2MB of memory was reserved using "no-map flag using DT". Unless this somehow translates to ARCH_PFN_OFFSET at build time, we would underflow mem_map, right? Maybe I'm just overly paranoid here and of course ARCH_PFN_OFFSET is determined properly on arm... If anyone can confirm my doubts or point me to what I'm missing, thanks.